AlCu and its related alloys are a preferred alloy for forming interconnections on electronic devices such as integrated circuit chips. The amount of Cu in AlCu is typically in the range from 0.3 to 4 percent.
Replacement of AlCu by Cu and Cu alloys as a chip interconnection material results in advantages of performance. Performance is improved because the resistivity of Cu and certain copper alloys is less than the resistivity of AlCu; thus narrower lines can be used and higher wiring densities will be realized.
The advantages of Cu metallization have been recognized by the entire semi-conductor industry. Copper metallization has been the subject of extensive research as documented by two entire issues of the Materials Research Society (MRS) Bulletin, one dedicated to academic research on this subject in MRS Bulletin, Volume XVIII, No. 6 (June 1993) and the other dedicated to industrial research in MRS Bulletin, Volume XIX, No. 8 (August 1994). A 1993 paper by Luther et al., Planar Copper-Polyimide Back End of the Line Interconnections for ULSI Devices, in PROC. IEEE VLSI MULTILEVEL INTERCONNECTIONS CONF., Santa Clara, Calif., Jun. 8-9, 1993, p. 15, describes the fabrication of Cu chip interconnections with four levels of metallization.
Processes such as Chemical Vapor Deposition (CVD) and electroless plating are popular methods for depositing Cu. Both methods of deposition normally produce at best conformal deposits and inevitably lead to defects (voids or seams) in wiring especially when trenches have a cross section narrower at the top than at the bottom as a result of lithographic or reactive ion etching (RIE) imperfections. Other problems of CVD have been described by Li et al., Copper-Based Metallization in ULSI Structures—Part II: Is Cu Ahead of its Time as an On-chip Material?, MRS BULL., XIX, 15 (1994). In electroless plating, while offering the advantage of low cost, the evolution of hydrogen during metal deposition leads to blistering and other defects that are viewed as weaknesses for industry wide implementation.
An electroplating process for depositing copper, silver or gold onto a semiconductor wafer is described in U.S. Pat. No. 5,256,274 ('274), which issued on Oct. 26, 1993, to J. Poris. In FIG. 1A of '274, a copper conductor is shown with a seam in its center with the legend “GOOD” and in FIG. 1B a copper conductor is shown with a void in its center with the legend “BAD.” The plating bath contained 12 ounces/gallon of water of CuSO4, 5H2O, 10% by volume of concentrated sulfuric acid, 50 parts per millions of chloride ion from hydrochloric acid, and TECHNI-COPPER W additive 0.4% by volume provided by Technic Inc., P.O. Box 965, Providence, R.I. 02901. Plating was selectively deposited through an inert mask.